1. Field of the Invention
The present invention relates to a fast atom beam source which is capable of emitting a fast atom beam efficiently.
2. Prior Art
Atoms and molecules subject to thermal kinetics in atmosphere at room temperature generally have a kinetic energy of about 0.05 eV. Atoms and molecules that fly with a much larger kinetic energy than the above are generally called a "fast atoms", and when a group of such fast atoms flow in the form of a beam in one direction, it is called "fast atom beam".
FIG. 2 shows one example of a fast atom beam source that emits argon atoms with a kinetic energy of 0.5 to 10 keV, among conventional fast atom beam sources designed to generate a fast beam of gas atoms. In the figure, reference numeral 1 denotes a cylindrical cathode, 2 a doughnut-shaped anode, 3 a DC high-voltage power supply of 0.5 to 10 kV, 4 a gas nozzle serving as a gas introducing means, 5 argon gas, 6 a plasma, 7 atom emitting holes, 8 a fast atom beam, and 9 a discharge stabilizing resistor.
The constituent elements, exclusive of the DC high-voltage power supply 3 and the discharge stabilizing resistor 9, are placed in a vacuum container. After the vacuum container has been sufficiently evacuated, the argon gas 5 is injected into the cylindrical cathode 1 from the gas nozzle 4. Meanwhile, a DC high voltage is impressed between the doughnut-shaped anode 2 and the cylindrical cathode 1 from the DC high-voltage power supply 3 in such a manner that the anode 2 has a positive potential, and the cathode 1 a negative potential. Consequently, a gas discharge occurs between the cathode 1 and the anode 2 to generate a plasma 6, thus producing argon ions and electrons. During this process, electrons that are emitted from the bottom surface 10 of the cylindrical cathode 1 are accelerated toward the anode 2 and pass through the central hole in the anode 2 to reach the bottom surface 11 at the other end of the cathode 1. The electrons reaching the bottom surface 11 lose their speed there. Then, the electrons turn around and are accelerated toward the anode 2. Thus, the electrons oscillate at high frequency between the two bottom surfaces 10 and 11 of the cylindrical cathode 1 through the central hole in the anode 2. While undergoing the high-frequency oscillation, the electrons collide with the argon gas to produce a large number of argon ions.
The argon ions produced in this way are accelerated toward the bottom surface 11 of the cylindrical cathode 1 to obtain a sufficiently large kinetic energy. The kinetic energy obtained at this time is about 1 keV when the voltage impressed between the anode 2 and the cathode 1 is, for example, 1 kV. The space in the vicinity of the bottom surface 11 of the cylindrical cathode 1 forms a turning point for electrons oscillating at high frequency, where a large number of electrons in a low energy state are present. Thus, argon ions that enter this region return to argon atoms through collision and recombination with electrons. In the collision between ions and electrons, since the mass of electrons is much smaller than that of argon ions so that it can be ignored, the argon ions deliver kinetic energy to the atoms without any substantial loss, thus forming fast atoms. Accordingly, the kinetic energy of the fast atoms is about 1 keV. The fast atoms are emitted in the form of a fast atom beam 8 to the out side through the atom emitting holes 7 provided in the bottom surface 11 of the cylindrical cathode 1.
In the conventional fast atom beam source shown in FIG. 2, however, since the electric line of force in the discharge region is not perpendicular to the cathode but is distributed in an irregular form due to the douhgnut-shaped anode and the cylindrical cathode, there is a problem that the directivity of the fast atom beam is not satisfactory. This problem is particularly pronounced when a fast atom beam having a large diameter is produced. In addition, the rate of neutralization varies with the change in the rate at which the gas is introduced into the cylindrical cathode 1. The rate of neutralization herein means the ratio of the number of neutralized fast atom particles to the total number of particles in the beam emitted. In the case of the conventional fast atom beam source shown in FIG. 2, the rate of neutralization is in the order of 30% to 60%.